The utility of solid-phase microextraction in evaluating polycyclic aromatic hydrocarbon bioavailability during habitat restoration with dredged material at moderately contaminated sites

The over- or underprediction of risk in moderately contaminated sediments can have a large impact on the nature of applied management strategies given that concentrations border on being toxic or not toxic. Project managers should give significant consideration as to how moderate levels of contaminants in native sediments and dredged material used for restoration will impact recovery of habitat. Total solid-phase (C_total) and porewater (C_free) polycyclic aromatic hydrocarbons (PAHs) were quantified in native sediments and dredged material to determine if the predictions of risk from C_total are consistent with those based on C_free. The sediment matrix phase in which PAHs were quantified resulted in disparate conclusions regarding the predicted reduction in contamination following restoration. Total solid-phase PAH concentrations suggested a significant decrease following restoration, whereas little to no change was observed in measured C_free. Risk metrics based on C_total gave inconclusive estimates for toxicity, whereas measured C_free suggested toxicity is unlikely, a conclusion consistent with toxicity testing. The incorporation of black carbon (BC) into model estimates for C_free gave predictions more consistent with measured C_free, suggesting that geochemical conditions (especially BC) play an important part in predicting toxicity at moderately contaminated sites. In addition to the use of C_free in toxicity evaluation, in-situ C_free measurements provided a constraint on diffusive PAH loads from sediment relative to ongoing stream loads. If passive sampling had been employed during the sampling designs and site evaluations, the costs of toxicity testing would not have been incurred, given that C_free suggested little to no toxicity. The results from the project highlight the benefits to be gained by moving beyond inconclusive, screening-level C_total metrics and implementing more sensitive and accurate C_free metrics in assessments of risk in moderately contaminated sediments. Integr Environ Assess Manag 2018;14:212–223.